Method for optimizing an image processing and printing process

ABSTRACT

A method for optimizing colored images emitted by a color printer on the non-white surfaces of substrates and for optimizing the amounts of printing ink used, wherein an image motif is processed by a computer-assisted image processing system in order to form a master copy which is ready for output. The method determines, for each pixel, whether and with what color density, a white underprint can be applied to a corresponding pixel, using an algorithm based on overall color density S F . The surface of the substrate is thus only underprinted with white on the pixels of the master copy where the overall color density is lacking or low.

[0001] The present invention relates to a method for optimising printimages output by colour printers onto substrate surfaces, in particularonto non-white substrate surfaces and for optimising the printing inkquantities used, an image motif being processed by means of acomputer-aided image processing system to form an image original whichis ready to output. The invention also relates to packaging filmsprinted by the method according to the invention.

[0002] Image motifs, with which a substrate surface is to be printed,are acquired or created by means of computer-aided image processingsystems and brought into image originals which are ready to output. Theprocessing of the image data into image originals which are ready tooutput takes place with the aid of appropriate image processingsoftware, such as for example PageMaker®, Quark Xpress®, Barco Packedge®or Macromedia Freehand® on DTP systems, wherein “DTP” stands for“Desktop Publishing”. DTP is a current designation for creating printpublications by means of computers. The image data are displayed on ascreen according to the principle of additive colour mixing, for examplein the known RGB format (R=red, G=green, B=blue).

[0003] The image original ready to output is then passed to an imageoutput system, converted into a format which can be read by an imageoutput system and printed by a colour printer, wherein when printing anon-white substrate surface, a white underprint is executed before theactual print of the image motif. If the printing is a counter-print,white is applied as the last colour, in other words after printing theactual image motif, as an overprint on the print image.

[0004] As the mode of operation of colour printers, such as for examplecolour printing systems operating by the electrophotographic method, isbased on the principle of subtractive colour mixing, the image data areconverted into a subtractive colour format prior to transfer to theimage output system. The known CMY colour space is generally used forthis purpose, comprising the three primary colour planes cyan (C),magenta (M) and yellow (Y). Cyan corresponds approximately to ablue-green and magenta approximately to a purple. The printing systemshere use a cyan, magenta and yellow printing ink, from which furthercolours can be produced, wherein the primary printing colours act ascolour filters. Light which falls through a C, M or Y primary printingcolour, is absorbed or filtered in certain spectral ranges by theprinting ink, so only light in a limited spectral range is reflected bythe printing ink, and perceived by the human eye as the colour of thetoner. Theoretically, black can be produced by the ideal mixing of theprimary printing colours C, M, and Y, as now all the light is absorbedor filtered. However, in practice a particularly deep and strong blackcannot be produced by mixing the primary printing colours, so apart fromthe CMY primary printing colours a black (K) printing ink completelyabsorbing the light is used for black portions and grey levels. Thecolour space supplemented by black is designated the CMYK colour space.

[0005] In digital image processing systems, the image original isdivided into individual image points, also called pixels. A respectivevalue for each of the four primary printing colours is allocated to eachimage point, for example by using the CMYK printing space. This valuerepresents the so-called colour density. For each image point variousmixed colours can be shown with the four colour planes and the colourdensity values allocated to them.

[0006] The colour density, also called colour covering, is astandardised variable for the applied quantity of printing ink. Thecolour densities F_(C) for C (cyan), F_(M) for M (magenta), F_(Y) for Y(yellow) and F_(K) for K (black) are in a defined range of 0 to 1 or 0to 100%, wherein 1 represents a maximum application of the correspondingprinting ink and 0 no application of the corresponding printing ink. Thesum of the individual applied colour densities are called the totalcolour density. The application of the maximum colour density F_(C),F_(M) and F_(Y) of the three primary colours CMY therefore produces thehigh total colour density or total colour covering of 3 or 300%.

[0007] The image data of the image original are either acquired in theform of raster or vector data. Accordingly, the image original can bepresent in a bitmap or vector graphics data file format. Standardisedvector graphics data file formats are, for example PostScript (PS) whichinter alia includes Encapsulated PostScripts (EPS) or Portable DocumentFormat (PDF). A standardised bitmap or raster graphics format is, forexample Tagged Image File Format (TIFF).

[0008] Generally the image originals which are ready to output areplaced in PostScript data files, these data files, apart from the actualimage data, containing further information necessary for furtherprocessing of the image data, for example with reference to formattingand instructions such as, for example, control instructions to the imageoutput system. Postscript data files may also contain inter alia imageobjects present in a bitmap format. The image original can therefore be,for example, an object embedded in the PostScript data file and presentin a raster graphics format, for example TIFF format.

[0009] Digital image output systems generally contain a “Raster ImageProcessor” (RIP) and a printing unit. The raster image processor (RIP)determines from the image original supplied, for example in a PostScriptdata file, the size, quantity and position of the image points (pixels)and converts these into a format which can be interpreted by a printingunit. The image data converted into printing instructions are convertedin the printing unit into a colour print.

[0010] The current image processing systems are designed for printingwhite substrate surfaces, in particular white paper. The white substratesurface is thus generally included in the colouring process in imageprocessing. White is for example generated by the allocation of thetotal colour density CMYK=0, in other words image points with thecorresponding zero value contain no colour application. Apart fromshowing white surfaces, white is also necessary for showing the colourbrightness. The colour brightness can be determined, on the one hand, byvarying colour densities and, on the other hand, by a raster display ofthe image points.

[0011] To obtain the same or a comparable colour impression whenprinting coloured, translucent or transparent substrates, as is producedduring printing of a white underlay, the substrate surface provided forprinting is therefore underprinted with white prior to the actualapplication of the print image. Underprint means that the white printingis located under the actual print image directly on the substratesurface. In the case of counter-printing on a transparent or translucentsubstrate with a coloured, translucent or transparent, in particularnon-white, substrate resting on the counter-print, a white overprint isapplied for the above-mentioned reasons directly on the print image.

[0012] However, in image regions with an adequately large total colourdensity, i.e. in image regions in which the white substrate surface isnot visible or does not shine through owing to a large colourapplication, a white underprint is not necessary. The dark colour tones,i.e. the places with a high total colour density, are frequently fadedor they even appear unsaturated owing to the white underprint, so inorder to achieve deep colours the overlying total colour density has tobe additionally increased.

[0013] Because of the surface-covering white underprint or overprint andthe print image arranged thereabove, a very high colour application isalso, moreover, often achieved and this can impair the melting of thetoner, for example, in the electrophotographic printing method and theimage quality.

[0014] Basically, the image processing can be oriented to the specificcolour properties of the substrate surfaces to be printed. However, thisrequires adaptation of the corresponding image processing systems, inparticular image processing software, connected with high expenditure.

[0015] The object of the invention is therefore to provide a method forcreating an image original which is ready for printing for visuallycoloured, translucent, transparent, specular or metallic appearingsubstrate surfaces or substrates and for the printing thereof, whereinthe drawbacks resulting because of the above-described white underprintor overprint are to be eliminated, without expensive adaptation of theimage processing software being necessary.

[0016] According to the invention, the object is achieved in that atimage points of the image original with small or missing total colourcovering, prior to application of the print image, a base print in whiteis applied directly to the substrate surface or in a counter-print, abase print in white is applied directly to the print image, wherein foreach image point at least the following steps are carried out in theshown indirect or direct sequence:

[0017] a) displaying the image data of the image original which is readyto output in a raster graphics format with the primary printing coloursCMYK;

[0018] b) applying a further colour plane “white” (W);

[0019] c) acquiring the colour density values F_(C) for C (cyan), F_(M)for M (magenta), F_(Y) for Y (yellow) and F_(K) for K (black) anddetermining an overall colour density S_(F) from the colour densityvalues F_(C), F_(M), F_(Y) and F_(K).

[0020] d) establishing the colour density value F_(W) for the colourplane white as a function F_(W)=f (S_(F)) of the total colour densityS_(F), wherein the dependency F_(W)=f (S_(F)) consists in the fact thatwith an increasing total colour density S_(F) over the interval of0≦S_(F)≦S_(Fmax) smaller colour density values F_(W) are continuouslyand/or discretely allocated, wherein S_(Fmax) corresponds to the maximumpossible total colour density S_(F),

[0021] e) creation of the modified image original in a raster graphicsdata format with at least 5 colour planes (CMYKW);

[0022] f) allocation of the modified image data of the image original toa printer driver or “Raster Image Processor” (RIP) and conversion into adata format which can be interpreted by the printer unit, printing theprint image and the white base print.

[0023] In a preferred embodiment of the invention, the colour densityvalue F_(W)=0 is allocated in the case where S_(F)=S_(Fmax). In afurther preferred embodiment of the invention the maximum colour densityvalue F_(W)=1 is allocated in the case where S_(F)=0.

[0024] In a development of the invention it is provided that at imagepoints of the image original with small or missing total colourcovering, prior to application of the print image, a base print in whiteis applied directly to the substrate surface, or, in a counter-print, abase print in white is applied directly to the print image, wherein foreach image point at least the following steps are carried out in theshown indirect or direct sequence:

[0025] a) displaying the image data of the image original which is readyto output in a raster graphics format with the primary printing coloursCMYK,

[0026] b) applying a further colour plane “white” (W),

[0027] c) acquiring the colour density values F_(C) for C (cyan), F_(M)for M (magenta), F_(Y) for Y (yellow) and F_(K) for K (black) allocatedto the individual image points,

[0028] d) allocation of weighting factors a, b, c and d with a valuefrom 0 to 1 and forming weighted colour density values: a*F_(C),b*F_(M), c*F_(Y) and d*F_(K),

[0029] e) forming a weighted total colour density S_(Fg) from the totalof the weighted colour density values:S_(Fg)=(a*F_(C)+b*F_(M)+c*F_(Y)+d*F_(K)),

[0030] f) establishing a critical total colour density SK and formingthe differential value D_(S)=S_(Fg)−S_(K) and establishing the intervallimits x, y with 0≦x≦(S_(Fmax)−S_(K)) and (0−S_(K))≦y≦0, wherein

[0031] i) a colour density value F of 0 is set for the colour planewhite in the case where D_(S)≧0 for x=y=0,

[0032] ii) a colour density value F_(W) of 1 is set for the colour planewhite in the case where D_(S)<0 for x=y=0,

[0033] iii) a colour density value F_(W) of 0 is set for the colourplane white in the case where D_(S)≧x for 0≦x≦(S_(Fmax)−S_(K)), iv) acolour density value F_(W) of 1 is set for the colour plane white in thecase where D_(S)≦y for (0−S_(K))≦y≦0, and

[0034] v) in the case where y<D_(S)<x for 0≦x≦(S_(Fmax)−S_(K)) and(0−S_(K))≦y≦0 a colour density value F_(W)=f (D) between 0 and 1 is setfor the colour plane white as a function f (D_(S)) to be determined,wherein F_(W)=f (D_(S)) describes a continuous increase in the colourdensity values F_(W) for reducing differential values D_(S),

[0035] g) creating the modified image original in a raster graphicsformat with at least 5 colour planes (CMYKW) and

[0036] h) allocating the modified image data of the image original to aprinter driver or “Raster Image Processor” (RIP) and conversion into adata format which can be interpreted by the printer unit, printing theprint image and the white base print.

[0037] If conventional printing is involved, i.e. not a counter-print, awhite underprint is applied by the printer unit directly onto thesubstrate surface following completed allocation of the modified imagedata of the image original to a printer driver or “Raster ImageProcessor” (RIP) and after conversion of the image original into a dataformat which can be interpreted by the printer unit with the aid of thecolour density values F_(W) for the colour plane white allocated to theindividual pixels. The print image is then printed on the whiteunderprint.

[0038] If the printing is a counter-print, once allocation of themodified image data of the image original to a printer driver or “RasterImage Processor” (RIP) has been completed and after conversion of theimage original into a data format which can be interpreted by theprinting unit, the print image is printed on the substrate surface. Awhite overprint is then applied to the print image with the aid of thecolour density values F_(W) for the colour plane white allocated to theindividual pixels. The so-called white base print is always behind theactual print image for the observer of the image print. Therefore, thebase print in conventional printing lies on the substrate surface andthe print image is arranged on the base print. On the other hand, with acounter-print the print image lies directly on the substrate surface ofa transparent or translucent substrate and the base print is arranged onthe print image. The print image can therefore be an underprint or anoverprint.

[0039] “Ready to output image original” means that the processing of theimage motif into an image original designed for the printing of whitesubstrate surfaces is complete. “Ready to print image original” meansthat the image original is ready for printing non-white substratesurfaces modified according to the invention and for transfer to animage output system.

[0040] Image motifs may, for example, contain drawing, letter or numbersequences, illustrations, patterns, rasters, random patterns orcombinations of said elements or may consist thereof. The printing imageis preferably a colour image.

[0041] With the inclusion of weighting factors a, b, c, d the differentvisual cooperation of the individual primary colours CMYK can be takeninto consideration with a white substrate surface. A colour applicationyellow (Y) in the maximum colour density of 1 may, for example, not yetcover the surface substrate in a visually opaque manner, while forexample, a colour application magenta (M) in the maximum colour densityof 1 may be able to cover the substrate surface in a visually opaquemanner.

[0042] In a particular embodiment of the invention there is, forexample, no weighting and the value 1 is allocated to the weightingfactors: a=b=c=d=1. The variables x and y establish interval limits forthe differential value D_(S)=S_(F)−S_(K). With the creation of thecondition 0≦x≦(S_(Fmax)−S_(K)) and (0−S_(K))≦y≦0 an interval in therange of y and x is formed around or at the differential value D_(S)=0which is produced from S_(F)=S_(K). According to the invention it isprovided that for differential values D_(S) lying within the interval,the colour density value F_(W) is determined for the function f (D_(S)),wherein the function f (D_(S)) for increasing differential values D_(S)describes continuously decreasing colour density values F_(W). Thefunction f (D_(S)) may be, for example, linear or non-linear, forexample exponential. Moreover, for differential values D_(S), which aregreater than or equal to x, the colour density value F_(W) is set at 0and for differential values D_(S) which are smaller or equal to y, thecolour density value F_(W) is set at 1. In cases iii) to v) only x or ycan equal zero, in each case.

[0043] In a particular embodiment of the invention no interval aroundthe differential value D_(S)=0 is provided. In this case x=y=0 is set.In the case for differential value D_(S) of greater or equal to 0 acolour density value F_(W) of 0 is set for the colour plane white and inthe case of a differential value D_(S) of less than 0 a colour densityvalue F_(W) of 1 is set for the colour plane white.

[0044] The weighting factors a, b, c, d, the critical total colourdensity S_(K) and the interval values x and y are preferably transferredas input variables to the computer unit or the programme routine by wayof appropriate input means.

[0045] Owing to the allocation according to the invention of a colourdensity F_(W) for each image point, no or one white base print with alow colour density F_(W) is executed, for example at image points with ahigh colour covering, which completely covers the white base print,whereas a white base print with a high or maximum colour density F_(W)is executed at image points with low or no colour covering.

[0046] It may be provided that, for example to include the visual effectof the substrate surface in the image design, a further colour planewith an imaginary special colour Z and a colour density F_(Z) of greaterthan 0 to 1 is allocated to image points with a total colour densityCMYK=0, i.e. image points without colour covering, but in which a whitebase print is dispensed with or the colour density F_(W) is to be lessthan 1. The colour density F_(Z) of the special colour Z represents thatproportion of the colour density F_(W) which is to be windowed in thewhite level so that:

F _(W)=1−F_(Z).

[0047] The creation of the colour plane Z and the allocation of thecorresponding colour densities F_(Z) preferably takes place during theimage processing, i.e. during the creation of the ready to output imageoriginal.

[0048] In a particular embodiment of the invention, a colour densityF_(Z) of either 0 or 1 is allocated for each image point with a totalcolour density CMYK=0 in a colour plane of the special colour, so awhite base print with a maximum colour density F_(W) is executed forF_(Z)=0 and no white base print is implemented for F_(Z)=1.

[0049] It can also be provided that the colour plane white is partiallyor completely windowed by allocation of a colour density F_(Z)>0 atimage points at which quite generally F_(W)>0. The total colour densityCMYK in these cases is 0 or greater, but less than the maximum totalcolour density.

[0050] The allocation according to the invention of the colour densityvalues F_(W) and optionally F_(Z) may also take place during the actualimage processing by means of desktop publishing, i.e. during thecreation of the ready to output image original. Furthermore, the methodsteps according to the invention may be a component of the imageprocessing software. In a preferred embodiment, the programme stepsaccording to the invention are carried out in a separate programmeroutine on the ready to output image original.

[0051] The ready to output image original may exist, for example, as aPostScript data file, the image data of the image original beingtransformed into a processable TIFF format to carry out the method stepsaccording to the invention. Once processing is complete the image dataare linked into the PostScript data file. The PostScript data file withthe ready to print image original modified according to the inventioncan then be passed to the image output unit.

[0052] The image data are preferably embedded as a raster graphicsobject in a PostScript data file. The raster graphics format ispreferably in TIFF format Version 6 or higher, i.e. in a version whichallows the creation of further colour planes apart from CMYK.

[0053] The printing unit of the image output system preferably operatesby an electrophotographic method using so-called toner systems. This maybe an indirect or direct electrophotographic method. An indirectelectrophotographic method is preferably used, such as for example isknown from xerography.

[0054] The toner used in electrophotographic methods may be in solidform, for example powdery, wax-like or resin-like or in liquid or pastyform. The toner may be, for example a two-component toner, such as drytoner, for example present in powder form or in liquid form.Furthermore, the toner in an expedient embodiment may also be a singlecomponent toner, made for example of resin particles, in which interalia pigments are dispersed. Preferred are single component toners inthe form of resin particles and in particular preferred are dry tonersin powder form. The toner is suitably a transparent or translucenttoner.

[0055] Electrophotographic methods and the toner systems used for themare described in more detail, for example, in “Ullmann's Encyclopedia ofIndustrial Chemistry, Sixth Edition, 1999, Electronic Release: Chapter2.1.1. Electrophotography”.

[0056] The method according to the invention is suitable for printingflexible packaging films, in particular packaging films with non-whitesurfaces. The packaging films may, for example be transparent,translucent, opaque and/or coloured or colour-penetrated. The packagingfilms may comprise coloured, specular or metallised surface layers andhave visually coloured, specular and/or metallised appearing surfaces.

[0057] The packaging films may be monofilms or film composites. Thepackaging films may be layers or films made of plastics material, suchas polyolefins, polyesters, polypropylenes, polyamides or made ofmetals, such as aluminium, iron, steel or alloys thereof, or made ofpulps, such as paper, glassine or cellulose film. Furthermore, thepackaging material may also comprise ceramic coatings.

[0058] The method according to the invention is in particular forprinting flexible transparent or translucent packaging films comprisingor made of plastics material and for printing flexible packaging foilsmade of metal, for example aluminium or flexible packaging films, forexample substantially made of plastics material, with visuallymetallised or specular appearing surfaces.

[0059] The thickness of the packaging material may, for example be 5 μmto 1,000 μm. Thicknesses of 7 to 200 μm are suitable.

[0060] The printing process may, for example be integrated as a methodstep in-line in the continuous production process of packaging materialsor packagings. The packaging film is preferably continuously printed ina continuous film printing plant. The colours magenta, cyan, yellow,black and white are printed in different sequence from case to case atprinting stations arranged consecutively in a printing plant or printingunit.

[0061] Owing to the method steps according to the invention asurface-covering white underprint or overprint with maximum colourdensity can be dispensed with in most cases. The white colour coveringcan instead be allocated in a targeted manner to the individual imagepoints. Toner is thus saved. Furthermore, the total colour covering isgenerally less and this has a positive effect on the print quality.

[0062] The invention will be described in more detail hereinafter withreference to the flow chart according to FIG. 1.

[0063]FIG. 1 shows a flow chart 1 of the algorithm for determining thecolour density F_(W) (colour plane white) of an individual image point.At the beginning in step 2, the weighting factors a, b, c and d and theinterval limits x and y and the critical total colour density valueS_(K) are acquired as input variables. The weighting factors are in arange of 0 to 1. The values for x and y are either both “zero” or are inthe range 0≦x≦(S_(Fmax)−S_(K)) or (0−S_(K))≦y≦0, wherein only x or y canbe “zero”.

[0064] For each colour plane, the colour density values F_(C), F_(M),F_(Y), and F_(Z) are determined in step 3. A weighted total colourdensity S_(F) is calculated in step 4. The differential value D_(S) isdetermined from the weighted total colour density S_(F) and the criticaltotal colour density value S_(K) in the following step 5. If x and yboth equal “zero” according to step 6, the colour density value F_(W)for white equals “zero” is set for a differential value D_(S) of greaterthan or equal to “zero” according to step 7 and the colour density valueF_(W) for white equals “one” is set for a differential value D_(S) ofless than “zero”. If neither x nor y equals “zero” there is an intervalaround the critical total colour density value S_(K). If thedifferential value D_(S) according to step 10 is greater than or equalto x the colour density value F_(W) for white is set at equals “zero”and if the differential value D_(S) according to step 12 is less than orequal to y, the colour density value F_(W) for white is set at equals“zero”. In all other cases the differential value D_(S) is in theinterval between y and x around the critical total colour density valueS_(K) and the colour density value F_(W) for white is described by alinear function F_(W)=f (D_(S)).

1. Method for optimising print images output onto substrate surfaces, inparticular onto non-white substrate surfaces by colour printers and foroptimising the printing ink quantities used, an image motif beingprocessed by means of a computer-aided image processing system to forman image original which is ready to ouput, characterised in that atimage points of the image original with small or missing total colourcovering, prior to application of the print image, a base print in whiteis applied directly to the substrate surface or, in a counter-print, abase print in white is applied directly to the print image, wherein foreach image point at least the following steps are carried out in theshown indirect or direct sequence: a) displaying the image data of theimage original which is ready to output in a raster graphics format withthe primary printing colours CMYK; b) applying a further colour plane“white” (W); c) acquiring the colour density values F_(C) for C (cyan),F_(M) for M (magenta), F_(Y) for Y (yellow) and F_(K) for K (black) anddetermining an overall colour density S_(F) from the colour densityvalues F_(C), F_(M), F_(Y), and F_(K). d) establishing the colourdensity value F_(W) for the colour plane white as a function F_(W)=f(S_(F)) of the total colour density S_(F), wherein the dependencyF_(W)=f (S_(F)) consists in the fact that with an increasing totalcolour density S_(F) over the interval of 0≦SF≦S_(Fmax) smaller colourdensity values F_(W) are continuously and/or discretely allocated,wherein S_(Fmax) corresponds to the maximum possible total colourdensity S_(F), e) creation of the image original in a raster graphicsdata format with at least 5 colour planes (CMYKW); f) allocation of themodified image data of the image original to a printer driver or “RasterImage Processor” (RIP) and conversion into a data format which can beinterpreted by the printer unit, printing the print image and the whitebase print.
 2. Method according to claim 1, characterised in that atimage points of the image original with small or missing total colourcovering, prior to the application of the print image, a base print inwhite is applied directly to the substrate surface or, in acounter-print, a base print in white is applied directly to the printimage, wherein for each image point at least the following steps arecarried out in the shown indirect or direct sequence: a) displaying theimage data of the image original which is ready to output in a rastergraphics format with the primary printing colours CMYK b) applying afurther colour plane “white” (W), c) acquiring the colour density valuesF_(C) for C (cyan), F_(M) for M (magenta), F_(Y) for Y (yellow) andF_(K) for K (black) allocated to the individual image points, d)allocation of weighting factors a, b, c and d with a value from 0 to 1and forming weighted colour density values: a*F_(C), b*F_(M), c*F_(Y),and d*F_(K), e) forming a weighted total colour density S_(Fg) from thetotal of the weighted colour density values:S_(Fg)=(a*F_(C)+b*F_(M)+c*F_(Y)+d*F_(K)), f) establishing a criticaltotal colour density S_(K) and forming the differential valueD_(S)=S_(Fg)−S_(K) and establishing the interval limits x, y with0≦x≦(S_(Fmax)−S_(K)) and (0−S_(K))≦y≦0, wherein i) a colour densityvalue F_(W) of 0 is set for the colour plane white in the case whereD_(S)≧0 for x=y=0, ii) a colour density value F_(W) of 1 is set for thecolour plane white in the case where D_(S)<0 for x=y=0, iii) a colourdensity value F_(W) of 0 is set for the colour plane white in the casewhere D_(S)≧x for 0≦x≦(S_(Fmax)−S_(K)), iv) a colour density value F_(W)of 1 is set for the colour plane white in the case where D_(S)≦y for(0−S_(K))≦y≦0, and v) in the case where y<D_(S)<x for0≦x≦(S_(Fmax)−S_(K)) and (0−S_(K))≦Y≦0 a colour density value F_(W=f (D)_(S)) between 0 and 1 is set for the colour plane white as a function f(D_(S)) to be determined, wherein F_(W)=f (D_(S)) describes a continuousincrease in the colour density values F_(W) for reducing differentialvalues D_(S), g) creating the modified image original in a rastergraphics format with at least 5 colour planes (CMYKW) and h) allocatingthe modified image data of the image original to a printer driver or“Raster Image Processor” (RIP) and conversion into a data format whichcan be interpreted by the printer unit, printing the print image and thewhite base print.
 3. Method according to either of claims 1 to 2,characterised in that the allocation of the modified image data of theimage original takes place on a printer driver or “Raster ImageProcessors” (RIP) and the image original is converted into a data formatwhich can be interpreted by the printer unit, and a white underprint isapplied by the printer unit to the substrate surface with the aid of thecolour density values F_(W) for the colour plane white allocated to theindividual pixels and the print image is then printed on the whiteunderprint.
 4. Method according to any one of claims 1 to 3,characterised in that the printing is a counter-print and the allocationof the modified image data of the image original takes place on aprinter driver or “Raster Image Processor” (RIP) and the image originalis converted into a data format which can be interpreted by the printerunit, and the print image is printed on the substrate surface and awhite overprint is applied by the printer unit to the print image withthe aid of the colour density values F_(W) allocated to the individualpixels.
 5. Method according to any one of claims 1 to 4, characterisedin that the image original is in a TIFF raster graphics format or isconverted into one.
 6. Method according to any one of claims 1 to 5,characterised in that a further colour plane is applied with animaginary special colour Z and a colour density F_(Z) of greater than 0to 1 is allocated to image points with a total colour density CMYK=0, inwhich a white base print is dispensed with or the colour density F_(W)should be smaller than 1, wherein the colour density value F_(Z)represents that part of the colour density F_(W) of the colour planewhite which should be windowed in the colour plane white, so that infulfilling the condition CMYK=0 and F_(Z)>0 the allocation: F _(W)=1−F_(z) applies.
 7. Method according to claim 6, characterised in that thecolour plane Z is allocated, and the colour density values F_(Z) areallocated, during the creation of the image original which is ready tooutput.
 8. Method according to claim 6, characterised in that the colourdensity F_(Z) can exclusively adopt the values 0 or 1, and the colourdensity value F_(W) can be set for F_(Z)=0 at 1 and for F_(Z)=1 at
 0. 9.Method according to any one of claims 1 to 8, characterised in thattransparent, translucent, opaque and/or coloured or colour-penetratedpackaging films are printed.
 10. Method according to any one of claims 1to 9, characterised in that flexible packaging films are printed withoptically coloured, specular and/or metallised appearing surfaces. 11.Transparent, translucent, opaque and/or coloured or colour-penetratedpackaging films, printed by a method according to claims 1 to
 10. 12.Flexible packaging films with optically coloured, specular and/ormetallised appearing surfaces, printed by a method according to claims 1to 10.